Forests serve many environmental functions aside from carbon mitigation. Natural forests with various stages of stand development, including old-growth forests with snags and fallen logs, provide diverse habitats necessary for biodiversity (Harris, 1984; Franklin and Spies, 1991). Stopping or slowing deforestation and forest degradation, therefore, not only maintains carbon stocks but also preserves biodiversity, as shown by studies in Belize (EPA/USIJI, 1998) and Paraguay (Dixon et al., 1993).
Although plantations usually have lower biodiversity than natural forests (Yoshida, 1983: Kurz et al., 1997; Frumhoff and Losos, 1998), they can reduce pressure on natural forests, leaving greater areas to provide for biodiversity and other environmental services (Sedjo and Botkin, 1997). Plantations can negatively affect biodiversity if they replace biologically rich native grassland or wetland habitat, but non-permanent plantations of exotic or native species can be designed to enhance biodiversity by stimulating restoration of natural forests (Keenan et al., 1997; Lugo, 1997; Parrotta et al., 1997a, 1997b). Measures to promote biodiversity of intensively managed plantations include the adoption of longer rotation times, reduced or eliminated clearing of understory vegetation, use of native tree species, and reduced chemical inputs (Allen et al., 1995; Da Silva Jr et al., 1995; Fujimori, 1997).
Preserving forests conserves water resources and prevents flooding. For example, the flood damage in Central America following hurricane Mitch was apparently enhanced by loss of forest cover. By reducing runoff, forests control erosion and salinity. Consequently, maintaining forest cover can reduce siltation of rivers, protecting fisheries and investment in hydroelectric power facilities (Chomitz and Kumari, 1996).
Afforestation and reforestation, like forest protection, may also have beneficial hydrological effects. After afforestation in wet areas, the amount of direct runoff initially decreases rapidly, then gradually becomes constant, and baseflow increases slowly as stand age increases towards a mature stage (Kobayashi, 1987; Fukushima, 1987), suggesting that reforestation and afforestation help reduce flooding and enhance water conservation. In water-limited areas, afforestation, especially plantations of species with high water demand, can cause significant reduction of streamflow, affecting inhabitants in the basin (Le Maitre and Versfeld, 1997). The hydrological benefits of afforestation may need to be evaluated site by site.
Forest protection may, however, have negative social effects, such as displacement of local populations, reduced income, and reduced flow of subsistence products from forests. Conflicts between protection of natural ecosystems and their other functions, such as production of food, fuelwood, and roundwood, can be minimized by appropriate land use on the landscape (Boyce, 1995; Forman, 1995) and appropriate stand management.
In arid and semi-arid regions, where deforestation is advancing (Kharin, 1996) and leading to carbon loss (Duan et al., 1995), restoring forests by afforestation and proper management of existing secondary forests can help combat desertification (Cony, 1995; Kuliev, 1996). Afforestation of desertified lands may be limited, however, by costs and insufficient knowledge of ecology, genetics, and physiology (Cony, 1995). In relatively arid regions, fuelwood plantations may reduce pressure on natural woodlands, thereby retarding deforestation (Kanowski et al., 1992).
Agroforestry can both sequester carbon and produce a range of economic, environmental, and socioeconomic benefits. For example, trees in agroforestry farms improve soil fertility through control of erosion, maintenance of soil organic matter and physical properties, increased N, extraction of nutrients from deep soil horizons, and promotion of more closed nutrient cycling (Young, 1997). Thus, agroforestry systems improve and conserve soil properties (Nair, 1989; MacDicken and Vergara, 1990; Wang and Feng, 1995). Examples of mitigation projects that promote soil conservation through agroforestry include the AES Thames Guatemala project, and the Profator project in Ecuador (Dixon et al., 1993; FACE Foundation, 1997).
We note that decisions to protect or enlarge forest cover on a large scale could also have secondary climate consequences through their feedbacks on the earths albedo, the hydrological cycle, cloud cover, and the effect of surface roughness on air movements (see, for example, Pielke and Avissar, 1990; Nobre et al., 1991; Garratt, 1993). Analyses by Bonan and Shugart (1992) suggest that large-scale changes in vegetative cover in the boreal zone may be especially important, with potentially global-scale impacts. In the boreal zone the albedo contrast between forested and unforested land during the winter is particularly large (differences as large as 40%). Indications are that the nature, magnitude, and even direction of climate changes driven by changes in surface vegetative cover will depend on the nature, location, hydrological setting, etc. of the vegetative change.
Other reports in this collection